Heat exchangers are well-known in the art and have been used in various forms for centuries to facilitate the transfer of heat from one medium to another. One class of heat exchangers, commonly referred to as boilers or vapor generators, is used to vaporize a working fluid. A portion of the heat used to vaporize the working fluid thereafter may be recovered from the vaporized working fluid to perform useful work. For example, boilers or vapor generators are commonly used in electrical power generation systems in which the working fluid, typically water, is heated in the boiler or vapor generator to produce steam, i.e., vaporized water. The steam is then expanded through a turbine or other such device in order to rotate an electrical generator. In the case of electrical power generation systems, the heat energy required to heat the working fluid may come from a wide variety of sources, including coal, natural gas, geothermal sources, and nuclear sources, although other heat sources may also be used.
While electrical generation systems of the type described above traditionally have used water as the working fluid (e.g., in the well-known Rankine cycle), newly developed thermodynamic cycles (e.g., any one of the so-called Kalina cycles) have been proposed that utilize “mixed” working fluids comprising two or more vaporizable components. The mixed component working fluid vaporizes and condenses progressively over a temperature range rather than at the relatively constant temperature of a so-called “pure” working fluid (e.g., water). Accordingly, thermodynamic cycles utilizing mixed working fluids can, if properly designed, realize increased efficiencies over similar thermodynamic cycles that utilize pure working fluids, such as water.
One design consideration for a thermodynamic cycle that utilizes a mixed working fluid relates to the heat exchanger utilized to transfer heat from the heating medium to the mixed working fluid. That is, since mixed working fluids vaporize over an increasing temperature range, it is generally preferred to design the heat exchanger so that heating function of the mixed working fluid closely follows the cooling function of the heating medium. For example, a primary consideration of geothermal power generation systems relates to the maximum brine flow-rate that can be extracted from the geothermal resource on a continuous or sustainable basis. Of course, regardless of the brine flow rate that can be extracted from a particular geothermal resource, a well-designed geothermal power generation system seeks to maximize the amount of useful work that can be generated from the particular brine flow rate.